As you saw from the last post, what makes the entire Pilatus experience (and business model) possible is the fantastically reliable Pratt and Whitney PT-6 engine. The PC-12NG is equipped with a PT6A-67P model engine. This is capable of about 1800 thermodynamic horsepower but is de-rated to 1200 shaft horsepower for normal operation on the PC12. It runs at a constant 1700 rpm in flight (yup, no rpm controller on a turbine) You just push that one big lever forward and make all the power you want. (and don’t forget to push lots of right rudder)
The technical term for the PT6 is reverse flow, radial inlet (with screen for foreign object damage protection), 4 stage, free turbine (two shaft) engine. The air intake is in the rear of the engine and flows forward toward the prop as it is compressed and ignited, incorporating 3-stage axial and 1-stage centrifugal compressor sections and two power turbines. The compressor turbines turn at around 35,000 rpm and are not mechanically connected to the power generating (propeller) side of the engine. These two shafts actually turn in opposite directions.
This video provides a very good overview of the parts of the engine:
You probably have seen a PT6 start on the ramp. The batteries (or external power) bring the engine up to a speed of about 13% NG to provide enough airflow so the fuel can be safely introduced without an overtemp. This is the only operator task on starting. At this point the engine picks up speed (sometimes with a little “poof” of smoke) and winds up to a “self-sufficient” (50%NG) speed where the generators cycle on line and the propeller comes out of feather. All this is all handled with one automated process. The only pilot task is hit “start” and introduce the fuel at 13%NG. At this point the pilot is only monitoring and watching for an overtemp or abnormal indications. If it is a hot engine being started, what you watch is decreasing ITT temp (from 200 or so depending on how long you have been sitting) down to 150 degrees ITT before introducing the fuel and starting the automated cycle.
As you can see, lots of moving parts and not much operator intervention in the production or maintenance of the power section. The incredible reliability is a result of a durable design and careful manufacturing process as well as constant monitoring and attention to detail. The website PT6Nation covers the engine development and history in detail (it is almost a cult). We are required to record engine trend once a day in cruise and the automated system downloads the data stream constantly for any abnormal indications to be detected.
This cut-away animation and description (OSH last year) gives you more of a complete idea of what is going on with the PT6 engine operation (and the basics of turbine mechanics) Follow the airflow…
I am continually amazed (and thankful) at the efficiency and reliability of this engine. I might have joined the PT6 cult (I should order the t-shirt!) Next time we will talk about cruise considerations for various missions we encounter. Our trip to Montana is pushing this airframe to it’s limit, we will see how that goes soon.